1. Field of the Invention
This invention relates to detectors and, more particularly, to the biasing of photoconductive detectors.
2. Background Art
The use of photoconductive detectors for measuring radiation is well-known. Because of their high sensitivity, photoconductive detectors such as PbS and PbSe are particularly effective in measuring infrared radiation. Detection of infrared radiation is used by the military for tracking warm vehicles and in night vision devices, is used by medical instrument manufacturers for measuring glucose and other body constituents in a noninvasive manner and is used by scientific instrument manufacturers for measuring chemical composition and structure.
In general, the resistance of the photoconductive detector changes when the radiation falls on its surface. Resistance changes can be measured as an electrical signal change and the intensity of the detected radiation can be estimated by the magnitude of resistance change.
Photoconductive detectors typically require a bias current or voltage to operate, such as a direct current bias. The sensitivity of the detector is proportional to the magnitude of the applied bias. It is preferred to supply a high bias to such a detector to increase its sensitivity and to overcome the noise of the electronics associated with the detector in an overall detection system.
The bias voltage applied to a photoconductive detector also causes current to flow in the absence of incident radiation. This current, referred to as the "dark current" or "dark signal", is usually large when compared to the current changes resulting from incident radiation. The detection of the small, radiation related signal, which is added to the large, dark signal, is often difficult. The electronics used with a photoconductive detector are designed to separate the desired radiation related signal from the undesired dark signal. The radiation related signal is usually a periodically modulated signal which can be separated from the DC dark signal with a DC blocking element such as a capacitor. In addition, the electronics used with a photoconductive detector are usually designed to improve the signal-to-noise ratio of the raw signal that is generated by the photoconductive detector. This signal-to-noise ratio can be improved by electronics that detect only the narrow band of frequencies of the radiation related signal while rejecting all other frequencies. As noise is spread across all frequencies, detecting only a narrow band of frequencies reduces that noise. A common method of reducing this noise is by the use of a lock-in amplifier, which is also referred to as a synchronous detector. This method can result in a DC output signal that is proportional to the magnitude of the radiation related input signal.
It is common to use a plurality of photoconductive detector elements, also referred to as pixels, in the form of a linear array mounted on a common substrate to measure radiation across a spectrum of wavelengths. Each detector element is responsive to and detects a particular wavelength, or band of wavelengths, of radiation. In applications such as infrared scanners and cameras, it is a goal to integrate as many photoconductive detector elements and their associated electronics into a single compact package. In such an arrangement, the synchronous detection method discussed above has not been used because the large number of electronic components needed to implement this method cannot be readily placed on a single package. For such synchronous detection, each photoconductive detector element requires a high pass filter, a preamplifier, a multiplier and an integrator. A multiplexer would typically follow the integrators so that the plurality of photoconductive detector elements would supply their DC output signals to an overall detection system through a single output pin.
One prior art method used for integrating a number of photoconductive detector elements and electronics into a compact package includes a dark signal subtraction circuit and a preamplifier for each detector element. Noise reduction methods are then applied external to the package. In applications where it is desired to integrate a plurality of photoconductive detector elements and electronics into a compact package, lock-in amplification has not been used. At the modulation frequencies used with typical photoconductive detector elements, the blocking capacitor must be large. For such lock-in amplification, each photoconductive detector element requires two amplifiers and a multiplexer. This arrangement results in a rather large component count for a compact package and is has not been used. Rather, the prior art has used the subtraction of the dark signal followed by amplification and integration and then multiplexing of the integrated signal from several detector elements to one output.
It is an object of the present invention to provide a synchronous photoconductive detection system which reduces the number of components needed for the overall system so that a system with a plurality of detector elements and associated electronics can be included on a single compact package and provide multiplexing of the signals generated by the detector elements.